The lost foam casting process is a well known method of producing metal castings of complex shape wherein an expanded polymeric foam pattern is embedded in a mold typically comprising unbonded foundry sand, and molten metal is poured into the mold to evaporate and displace the pattern in the mold. The dimensions of the casting closely reflect the original dimensions of the foam pattern that the metal replaces. Thus, it is important to use patterns as dimensionally accurate and stable as possible.
The polymeric foam pattern is obtained by molding pre-expanded polystirene or other polymer beads in a pattern mold to impart the desired configuration to the pattern. For example, a commonly used material for making polymeric foam patterns comprises expandable polystirene (EPS) raw beads that contain a blowing agent that typically includes mostly normal pentane with other alkanes also present (e.g. some raw beads are supplied with up to about 30% by weight alkanes other than normal pentane) and that have a raw bead size distribution with over 90% of the beads having a bead diameter in the range of about 0.2 to 0.5 millimeters. These are referred to as T-beads and are needed to provide a satisfactory pattern surface and to allow formation of thin-walled patterns, such as for example only, patterns with wall thicknesses of approximately 3 to 5 millimeters for casting vehicle engine blocks. The EPS raw beads are pre-expanded at a temperature above the softening temperature of polystirene and above the boiling point of the blowing agent. The pre-expanded EPS beads then are molded into the desired configuration in a pattern mold that is steam heated to further expand the beads and then water cooled to stop the expansion process after the pattern is formed to shape. The pattern then is removed from the mold.
Upon removal from the mold into the ambient atmosphere, such polystirene foam patterns are known to initially grow in size as air diffuses into the pattern and then to shrink in size. In the past, conventional lost foam casting practice involved storing the molded polystirene foam patterns for an extended amount of time (e.g. 30 days) at room temperature until dimensional equilibrium was approached, and then to proceed with use of the patterns in casting molten metal. Another practice involves preexpanding EPS beads, molding the beads to form a desired pattern, and then subjecting the pattern to oven aging to rapidly bring the pattern to stable dimensions.
A method to more rapidly stabilize pattern dimensions is described in U.S. Pat. No. 5,385,698 where pre-expanded EPS beads are expanded from dense raw beads and heated for a time prior to molding to form a desired pattern.
Another attempt to rapidly dimensionally stabilize the patterns described in U.S. Pat. No. 4,816,199 involves pre-expanding EPS beads, molding the expanded beads to form a desired pattern, and then subjecting the molded pattern to subatmospheric pressure in the range of 2-20 inches Hg for at least 5 hours to rapidly bring the pattern to stable dimensions.
An object of the present invention is to provide a method of lost foam casting using an improved dimensionally self-stabilized polystirene foam pattern that permits direct use of the pattern in the lost foam casting of molten metal without the need for the post-molding, dimension-stabilizing pattern treatments described above.